Seed plants were the most recent evolutionary step


The big picture: We now live in a seed plant world



  • Seeds changed the course of plant evolution
    • Gymnosperms evolve ~360 mya
    • climate became drier = selection pressures
    • Elkensia first fossil seed plant


  • Seeds consists of embryo & nutrients
    • surrounded by protective coat
    • seeds can be dispersed long distances
    • less dependence on water


  • 2 living seed lineages
    • gymnosperms (naked seed)
    • angiosperms (covered seed = fruits)

Why seeds matter so much…



  • Before seeds, spore was only protective structure
    • embryo exposed


  • Seeds are multicellular
    • what can this add?


  • Seeds can remain dormant for a long time


  • Gymnosperms: seeds exposed on sporophylls, form cones
  • Angiosperms: seeds enclosed inside fruits

Seed plants further adapt to life on land


  • Reduction of gametophyte
    • mosses → ferns → seed plants
    • gametophyte become microscopic


  • Heterosporous life cycle
    • 2 kinds of spores
    • form male/female gametopytes


  • Ovules
    • megasporangium kept inside sporophyte
    • ovule contains megaspore


  • Pollen
    • microspore develops into pollen grain

4 living lineages of gymnosperms


  • Cycads: 300 living species
    • earliest evidence of insect pollination
    • Cycads and dinosaurs lived together


  • Gnteophytes: small and super weird group
    • some flowering traits


  • Ginkos: Ginkgo biloba is only species left!
    • landscapers only plant males because…


  • Conifers: ~600 species (cones)
    • evergreen and deciduous
    • leaves are needles or scales

Welwitchia: the weirdest f’in plant on Earth


Gymnosperm reproduction: basics




  • Male and female cones on same plant
    • sporophyll → sporangia → spores
    • scales → modified leaves → sporophyll
    • pollen (male) and ovulate (female) cones


  • Male cones house microsporangium
    • microsporangium produces microspores
    • microspores develop into pollen grains
    • male gametophyte inside pollen grain
    • pollen dispersed by wind

Gymnosperm reproduction: basics




  • Female cones have 2 ovules per scale
    • each ovule has megasproangium
    • megasporangium produce 4 megaspores
    • 1 megaspore survives


  • Pollen grain reaches ovule and germinates
    • pollen tube grows toward megaspore
    • megaspore develops gametophyte with egg
    • sperm → pollen tube → egg
    • seed forms, dispersed by wind

Flowering plant diversity



  • Most diverse living group of plants
    • 14,000 genera, 350,000 species
    • 94% of plant diversity
  • 2 classes: Moncots and Dicots
    • monocots = one seed leaf in embryo
    • dicots = two seed leaf in embryo

Angiosperm evolution: plants and insects


Angiosperm reproduction: Flowers



  • Flower is a modified stem for reproduction
    • with 4 types of modified leaves


  • Sepals: leaves at base which enclose flower
    • usually green


  • Petals: colored leaves to attract pollinators
    • less colored if wind pollinated


  • Two types of fertile organs to make spores
    • sporophylls (male and female)

Flowers - Male




  • Stamens: microsporophylls
    • produce microspores that develop into pollen


  • Anther: where microsporangia develop
    • microspores produced
    • microspores → pollen
    • pollen → male gametophyte


  • Filament: stalk that holds the anther

Flowers - Female



  • Carpel: megasporophyll


  • Stigma: receives pollen
    • sticky


  • Style: leads from stigma to ovary
    • where pollen tube forms


  • Ovary: contains ovules
    • megasporangium inside ovule produce 4 megaspores
    • 1 megaspore survives to make female gametophyte

Angiosperm reproduction: double fertilization


Comparing seed plant reproduction